Being Thai, I'm pretty sure that the resemblance is merely coincidental. The words for "fish", and "swim" in Thai are:
fish: ปลา
[plaː]
swim: ว่าย[wâːi]
Thai has a word for "fish" that was borrowed from Pali/Sanskrit:
มัจฉา
[mát-tɕʰǎː]
which sounds not very close to the mentioned Slavic word.
I'm more curious in how words like "elephant", "avatar", "immortal", which Thai borrowed from Pali/Sanskrit as เอราวัณ [eː-raː-wán] อวตาร [à-wá-taːn] อมตะ [à-má-tàʔ], sound like in Slavic. (NOTE: The more common word for elephants, which is not a borrowed word, in Thai is ช้าง [tɕʰáːŋ].)
